The effect of the striated muscle fiber membrane potential on sodium and potassium ion active transport rates will be determined. The membrane potential will be changed by two means: "chloride-clamping" of the membrane potential at about -20 mV by prior equilibration of muscles in 95 mM KCl Ringer solution and also by external application of sodium azide. Sodium efflux and potassium influx will be measured both in the presence and absence of ouabain to deduce active components of ion flux at different membrane potentials. In addition, the sensitivity of outward sodium ion transport to external potassium ions will be assessed at the different membrane potentials. The relative roles of membrane potential and (K)o in regulating the rates of Na and K ion transport will thus become evident. In the case of passive potassium ion fluxes, experiments will be performed in the presence of ouabain and the membrane potential will be altered by application of sodium azide. By doing experiments at various values of (K)o, it is hoped that the effect of the membrane potential on passive K ion fluxes can be separated from the effect of (K)o. In addition, an experimental program on Na and K ion fluxes in injected and dialyzed myxicola giant axons will be initiated.